1. Field of the Invention
The present invention relates to a pneumatic tire (hereinafter referred to as a “tire”). More specifically, the present invention relates to a structure of a tire in which a tread has a two-layer structure.
2. Description of the Related Art
As shown in FIG. 4, a tire 1 comprises a carcass 2a, a belt 2b for reinforcing the carcass 2a, a sidewall 2c, a bead 2d, and a tread 3. The tread 3 includes a tread rubber layer 3a provided to cover the outer peripheral surfaces of the carcass 2a and the belt 2b. The outer peripheral surface of the tread rubber layer 3a constitutes a tread surface. A buttress portion 7 is provided in a boundary part between the tread rubber layer 3a and the sidewall 2c. The tread rubber layer 3a is generally constituted by a different kind of rubber from rubbers constituting the carcass 2a and the sidewall 2c. The tread rubber 3a is provided with a plurality of grooves 4 in a circumferential direction. As a result of the formation of the grooves 4, a tread pattern is formed on the outer peripheral surface of the tread rubber layer 3a. 
In consideration of the durable year of the tire 1, a thickness a of the tread rubber layer 3a is to be a certain thickness. The reason is that the tread rubber layer 3a is provided to cover the outer peripheral surfaces of the carcass 2a and the belt 2b, and furthermore, the groove 4 is formed. More specifically, it is necessary to set a depth c of the groove 4 to be a certain depth or more. If the depth c of the groove 4 is set to be great, however, it is a matter of course that the thickness a of the tread rubber layer 3a is increased. As a result, the weight of the tire 1 is increased, and furthermore, the high-speed durability of the tire 1 or the like is deteriorated.
In recent years, a tire to be attached to a vehicle with an increase in the speed of a vehicle has required a high durability at a high speed. In order to enhance the durability at a high speed, conventionally, the following means has been employed: (1) the thickness a of the tread rubber layer 3a is set to be small; (2) a band for reinforcing the belt 2b is provided; and (3) the material of a rubber constituting the tread rubber layer 3a is of a low heat generation type. According to the means of (1), the amount of heat generated from the tread 3 during running is decreased. Consequently, high-speed running can be continuously carried out. According to the means of (2), the lifting of the belt 2b and the carcass 2a in the running is prevented. Consequently, the high-speed running can be continuously carried out. According to the means of (3), the amount of heat generated from the tire during the running is decreased. Consequently, the high-speed running can be continuously carried out.
In the case in which the means of (1) is employed, however, it is necessary to ensure the depth c of the groove 4 to be a certain depth or more in order to maintain the durable year of the tire 1. Consequently, a dimension b between the bottom of the groove 4 and a tire body is decreased. If the dimension b is decreased, a ride comfort is deteriorated and a noise is increased. If the dimension b is decreased, furthermore, a crack is apt to be generated on the bottom of the groove. On the other hand, in the case in which the means of (2) is employed, the weight of the tire 1 is increased. Moreover, the ride comfort is deteriorated and the noise is increased. Furthermore, the rolling resistance of the tire 1 is increased. In the case in which the means of (3) is employed, the grip force of the tire 1 is reduced so that a running stability is deteriorated and a braking performance or the like is degraded.
Under the circumstances, conventionally, the tread rubber layer 3a has a two-layer structure including an outer layer 5 and an inner layer 6 in order to enhance a durability at a high speed while maintaining the durable year of the tire 1. The outer layer 5 (hereinafter referred to as a “cap layer”) is formed of a rubber having a high gripping property and the inner layer 6 (hereinafter referred to as a “base layer”) is formed of a rubber of a low heat generation type.